1. Field of the Invention
The present invention relates to a method for forming an insulating film and a semiconductor device and, more particularly, a method for forming an insulating film used as an interlayer insulating film of a high integration density semiconductor integrated circuit device and having a low relative dielectric constant and a semiconductor device employing the insulating film therein.
2. Description of the Prior Art
In recent, reconsideration of interlayer insulating films for use in multilayered interconnections has been effected from a view point of high integration density or high speed operation of semiconductor integrated circuit devices.
More particularly, from view points of stability, easiness of film forming, etc., an SiO2 film has been widely used as an interlayer insulating film of the semiconductor integrated circuit device. But the problem of increase in parasitic capacitance has arisen according to progress of recent miniaturization of the circuit.
Conventionally, a low temperature film forming method has been used in forming the interlayer insulating film when taking influence on aluminum interconnection layers, etc. into consideration. For instance, the methods for forming the SiO2 film employing a plasma chemical vapor deposition method (referred to as PCVD method hereinafter) using tetraethylorthosilicate (referred to as TEOS hereinafter), i.e., (C2H5O)4Si have been proposed (see Patent Application Publication (KOKAI) 6-240459 and Patent Application Publication (KOKAI) 6-140386).
In the former (Patent Application Publication (KOKAI) 6-240459), when the SiO2 film is formed in the course of manufacturing a polysilicon thin film transistor (poly-SiTFT) which is used in view finder being requested to have a large area over 300 mm, CCD, liquid crystal projector and the like, the ratio of the flow rate of O2 gas or mixed gas of O2 and He to TEOS is set at more than 50 times. Impurities cannot stay even in the center of the large area substrate, and can therefore be removed from a surface of a substrate in the course of manufacturing to thus result in uniform film quality.
However, in the foregoing forming method of the SiO2 film, step coverage of the interlayer insulating film is lowered because of wiring steps. As a result, the foregoing forming method of the SiO2 film is not suitable for forming the interlayer insulating film of monolithic semiconductor integrated circuit such as memory, logic circuit, and the like.
In the latter (Patent Application Publication (KOKAI) 6-140386), it has been intended to reduce humidity absorption which causes degradation of device characteristics. The ratio of the flow rate of O2 to TEOS has been set at about 2.2 times, i.e., the flow rate of O2 has been set at 730 cc/min in contrast to the flow rate of TEOS of 330 cc/min.
However, since the SiO2 film formed by the PCVD method has high relative dielectric constant such as about 4.1, parasitic capacitance between interconnection layers is relatively large. When distances between interconnection layers are narrowed according to the progress in miniaturization of the circuit, parasitic capacitance has been further increased. This parasitic capacitance causes signal propagation delay, so an operation speed cannot be improved not to respond to miniaturization of the devices.
In other words, if relative dielectric constant of the interlayer insulating film is k and resistance of the interconnection layer is R, wiring delay time xcfx84 caused by the interconnection layers is in proportion to kR. Accordingly, it would be understood that material having small relative dielectric constant may be used as the interlayer insulating film to improve the wiring delay time. Parasitic capacitance can be reduced according to reduction in relative dielectric constant to thus decrease signal propagation delay.
Recently, as a method for forming an insulating film with low relative dielectric constant, several methods for forming the SiO2 film including fluorine, i.e., SiOF film by adding fluorine contained gas into material gas in terms of the PCVD method have been reported (see Fukada, Akahori, Extended Abstracts of the 1993 International Conference on Solid State Devices and Materials, Makuhari, 1993, pp.158-160; Usami, Shimokawa, Yoshimura, Extended Abstracts of the same, pp.161-163; and Mizuno, Hara et al., Extended Abstracts of the same, pp.510-512). The SiOF film is worthwhile to be used as interlayer insulating film of the next generation semiconductor integrated circuit device since its relative dielectric constant is lower than 4.1 in the conventional SiO2.
However, the SiOF film has unstable relative dielectric constant because of its high humidity absorption, like the SiO2 film formed by the PCVD method. In other words, if the SiOF film having low relative dielectric constant can be formed at first, the relative dielectric constant is increased as time elapsed. This is because water component absorbed into the SiOF film has significantly high relative dielectric constant like 80. Such water absorption into the SiOF film has resulted in not only increase in the relative dielectric constant but also bad influence on the semiconductor device per se, so that reliability of the semiconductor device is lessened.
Currently, as the insulating film having lower relative dielectric constant than the above insulating film, the insulating film formed of fluorine macromolecule has been known in the art. For example, relative dielectric constant of polytetrafluoroethylene (P-TFE) is 2.2 (1 MHz). Since the value is smallest in organic materials, P-TFE is promising as the insulating film.
However, since fluorine macromolecule is in general insoluble into most solvents, they cannot be treated by spin coating or press coating as in ordinary photosensitive resins. So they have such drawbacks that it is difficult to form thin films by fluorine macromolecule.
In addition, although some of fluorine macromolecule materials which can be treated by spin coating are in market as a merchandise, they have poor adhesiveness and heat resistance. As a result, they have such drawbacks that decomposition or exfoliation from the substrate occurs in heating the substrate.
Besides, to overcome the problem of such adhesiveness, there are some reports wherein P-TFE thin film has been formed by plasma polymerization. However, the plasma polymerization film can provide good adhesiveness, but it has the drawback that heat resistance is poor. Furthermore, since polymer of low molecular weight is included into the film and released from the film when the substrate is heated, merely the film having relative dielectric constant such as about 2.7 (1 MHz) can be obtained.
For example, the following method has been proposed by Kazuhiko Endo and Toru Tatsumi. That is, using the parallel plate type plasma CVD equipment (CCP=Capacitive Coupled Plasma) and the helicon wave plasma CVD equipment, amorphous carbon fluoride film (fluorine type resin film) has been formed under the conditions that CF4+CH4 or C2F6+CH4 is used as material gas and substrate temperature is at 50xc2x0 C. (see Material Research Society, symposium proceedings, Vol.381, entitled Low-Dielectric Constant Materials-Synthesis and Applications in Microelectronics, 1995, pp.249 to 254).
According to this method, the fluorine type resin film enabling excellent heat resistance would be obtained by mixing CH4 as the hydrogen containing compound into the film when forming the film. More specifically, in stacking the film, fluorine (F) radical serving as the basis of etching operation is gettered by hydrogen (H) to reduce its etching operation and at the same time to produce carbon rich state so as to enhance bridging density. The fluorine type resin film having relative dielectric constant of 2.1 could be obtained by the parallel plate type plasma CVD equipment, and also the fluorine type resin film having relative dielectric constant of 2.4 could be obtained by the helicon wave plasma CVD equipment.
However, there has been caused the problem that the relative dielectric constant is increased to 2.7 by annealing process at 300xc2x0 C.
According to the film forming method excluding the film forming method using the helicon wave plasma CVD equipment and employing C2F6+CH4 as material gas, there has been caused the problem that residual film rate is lessened less than 70% by annealing process at 300xc2x0 C. for one hour to render heat resistance worse.
On the other hand, according to the film forming method using the helicon wave plasma CVD equipment and employing C2F6+CH4 as material gas, residual film rate becomes 100% after annealing process at 300xc2x0 C. for one hour, but residual film rate is lessened less than 60% by annealing process at 400xc2x0 C.
Similarly, the following method has also been proposed by Kazuhiko Endo and Toru Tatsumi. That is, using the parallel plate type plasma CVD equipment and employing CF4+CH4 added by N2 as material gas; amorphous carbon fluoride film (fluorine type resin film) has been formed at substrate temperature of 50xc2x0 C. (see Extended Abstracts of the 1995 International Conference on Solid State Devices and Materials, Osaka, 1995, pp.177 to 179).
In this case, fluorine type resin film having relative dielectric constant of 2.5 could be obtained. The relative dielectric constant remains at 2.5 and is scarcely changed after annealing process at 300xc2x0 C. for one hour, but there have been drawbacks that residual film rate is lessened less than 90% and in addition heat resistance becomes poor.
In two foregoing reports as for the amorphous carbon fluoride film (fluorine type resin film), annealing temperature denotes heater temperature. It may thus be supposed that actual temperature of the amorphous carbon fluoride film would be considerably lower than the heater temperature.
In the meanwhile, it can be supposed that characteristics which are required for the interlayer insulating film in the high integration density semiconductor device to be formed using a 0.18 xcexc/m design rule are that the relative dielectric constant is less than 2.5 and residual film rate is almost 100% after heat treatment at more than 300xc2x0 C.
However, these characteristics are difficult to be satisfied by the amorphous carbon fluoride film formed by the above two film forming methods.
It is an object of the present invention to obtain a PCVD-SiOF film which has stable characteristics and has low relative dielectric constant without applying low frequency power which affects transistor characteristics, from a view point specifying particular kinds of material gases and particular flow rate ratio.
It is another object of the present invention to obtain a PCVD-SiOF film which has excellent characteristics with good reproducibility by finding conditions permitting stable characteristics and lower relative dielectric constant from view points different from specifying particular kinds of material gases and particular flow rate ratio, on the basis of analysis of changing mechanism of relative dielectric constant in the PCVD-SiOF film.
It is still another object of the present invention to provide an interlayer insulating film formed of fluorine type resin film which has low relative dielectric constant, excellent heat resistance and can be formed as a thin film.
According to a method for forming an insulating film in terms of a plasma chemical vapor deposition method of the present invention, the PCVD-SiOF film which has stable characteristics and has low relative dielectric constant can be obtained by using tetraethylorthosilicate, oxygen and C2F6 as material gases, and setting a ratio of flow rate of oxygen gas to the tetraethylorthosilicate within a range between 20 or more and 40 or less.
The lower limit of the ratio of flow rate of 20 times is determined correspondingly to stability in a change with the passage of time. If the lower limit is less than 20 times, the change with the passage of time is enhanced even if the relative dielectric constant is small such as about 3.5 to 3.7 when being formed. There is no difference between this film and the conventional PCVD-SiO2 film. The upper limit of the ratio of flow rate of 40 times is determined correspondingly to film forming rate in practical level and step coverage for wiring steps. If the upper limit exceeds 40 times, film forming rate is reduced and step coverage for wiring steps is degraded. So the range of 20 to 40 is preferable.
In addition, the PCVD-SiOF film which has less hydrophilicity, more stable characteristics and lower relative dielectric constant can be obtained by using tetraethylorthosilicate, oxygen and C2F6 as material gases, and setting a ratio of flow rate of C2F6 to the tetraethylorthosilicate within a range between 5 or more and 7 or less.
The lower limit of the flow rate ratio of 5 times is determined correspondingly to resultant dielectric constant. If the lower limit is less than 5 times, relative dielectric constant is more than 3.7 when the film is being formed and it is more than 3.8 if a change with the passage of time thereafter is considered. Therefore, advantage of preventing signal propagation delay is lessened.
The upper limit of the flow rate ratio of 7 times is determined correspondingly to stability in a change with the passage of time. If the upper limit is more than 7 times, for example, 8 times, relative dielectric constant is very low such as 3.5 when the film is being formed. But the relative dielectric constant is increased to about 4 after several days. There is no difference between this film and the conventional PCVD-SiO2 film.
In the foregoing description, the excellent PCVD-SiOF film has been obtained by specifying material gases and manufacturing conditions. But by analyzing from another view point, i.e., from relation to characteristics of the PCVD-SiOF film, the invention in the following has been derived.
According to another method for forming an insulating film in terms of a plasma chemical vapor deposition method of the present invention, the PCVD-SiOF film which has stable characteristics can be obtained by using Si supply gas (33, 16 in FIG. 12), oxygen supply gas (13, 27 in FIG. 12) and fluorine supply gas (10, 30, 36 in FIG. 12) as material gases to form the insulating film having density of more than 2.25 g/cm3.
In other words, in case density of the PCVD-SiOF film is more than 2.25 g/cm3 dependency of film density on the flow rate can be decreased significantly, and therefore the PCVD-SiOF film having low relative dielectric constant of 3.6 or so can be obtained with good reproducibility even when manufacturing conditions are slightly varied. On the contrary, in case density of the PCVD-SiOF film is less than 2.25 g/cm3, film density may be changed significantly depending on manufacturing conditions such as the flow rate, and at the same time relative dielectric constant is changed largely to thus reach 3.7 or more finally.
In addition, the PCVD-SiOF film which has stable characteristics can be obtained by using a combination of the tetraethylorthosilicate as Si supply gas (16 in FIG. 12), oxygen as oxygen supply gas (13 in FIG. 12) and C2F6 as fluorine supply gas (10 in FIG. 12) as material gases to form the insulating film having density of more than 2.25 g/cm3.
Furthermore, the PCVD-SiOF film which has stable characteristics can be obtained by using SiH4 as Si supply gas (33 in FIG. 12), N2O as oxygen supply gas (27 in FIG. 12) and C2F6 as fluorine supply gas (10, 30, 36 in FIG. 12) as material gases.
Besides, the PCVD-SiOF film which has stable characteristics can be obtained by using another combination, i.e., SiH4 as Si supply gas (33 in FIG. 12), N2O as oxygen supply gas (27 in FIG. 12) and SiF4 as fluorine supply gas (36 in FIG. 12) as material gases.
As stated above, by providing the SiOF film having density of 2.25 g/cm3, the semiconductor device capable of reducing signal propagation delay can be derived.
According to the method for forming the interlayer insulating film of the present invention, as shown in FIG. 22, the fluorine type resin film 4 having high bridging density and excellent heat resistance because of being carbon rich can be obtained by effecting the plasma chemical vapor deposition method in a state wherein the hydrogen containing compound gas is mixed into fluorocarbon gas having the C/F ratio of 1/2.
If the ratio of C becomes higher, bridging density is increased and heat resistance can be improved, but relative dielectric constant is rendered high. Alternatively, if the ratio of F becomes higher, relative dielectric constant is rendered low, but reproducibility of the film is lessened by etching operation of F radical and heat resistance is lowered. Therefore, as the fluorocarbon being easily available, the one having the C/F ratio of 1/2 is suitable.
In the foregoing, by employing any one of C2F4, C3F6 and C4F8, especially C4F8 as the fluorocarbon gas, the interlayer insulating film having low relative dielectric constant such as relative dielectric constant of less than 2.5 and good residual film ratio can be achieved.
In addition, by employing any one of H2, SiH4, C2H2, C3H8, C2H6 and CH4 as the hydrogen containing compound gas, the interlayer insulating film having low relative dielectric constant such as relative dielectric constant of less than 2.5 can be achieved. The relative dielectric constant is increased with the flow rate of the hydrogen containing compound. Especially, by employing any one of C2H2 and C3H8 as the hydrogen containing compound gas, the interlayer insulating film having low relative dielectric constant such as less than 2.5 and excellent heat resistance can be achieved.
Further, if the film forming temperature in the plasma chemical vapor deposition step is set at more than 250xc2x0 C., the fluorine type resin film 4 having high bridging density and excellent heat resistance because of being carbon rich can be achieved.
Furthermore, according to the semiconductor device of the present invention, the interlayer insulating film is provided therein formed by any one of the above forming methods. In other words, if the fluorine type resin film 4 having low relative dielectric constant and excellent heat resistance is used as the interlayer insulating film, wiring delay time on the interconnection layer 3 of the high integration semiconductor device can be reduced significantly, and the reliability can be improved.